Hydrolyzed Yeast Supplementation in Calf Starter Promotes Innate Immune Responses in Holstein Calves under Weaning Stress Condition

SIMPLE SUMMARY: New-born Holstein calves are vulnerable to infectious diseases owing to incomplete immune system development. This study aimed to evaluate the effects of six weeks of hydrolyzed yeast supplementation in Holstein calf starter on stress and immune responses after weaning. Hydrolyzed yeast supplementation facilitated the innate immune response after weaning, with a limited stress response. Therefore, the supplementation of hydrolyzed yeast in calf starters may help weaned calves adapt to diet-induced stresses. These results suggest that hydrolyzed yeast may be an effective feed additive for modulating stress-related immune responses. ABSTRACT: Weaned calves are susceptible to infectious diseases because of the stress and malnutrition that occurs during weaning. Therefore, the dairy industry requires effective feed additives to ameliorate stress responses and promote immunity. This study aimed to investigate the effects of hydrolyzed yeast (HY) supplementation on the growth performance, immune and stress parameters, and health status of calves after weaning. Eighteen Holstein calves were randomly assigned to two groups, either receiving a control calf starter or 0.2% HY calf starter from one week of age. All calves were weaned at six weeks of age as a stress challenge. The HY-fed calves had a significantly-higher body weight gain during the post-weaning period (kg/week) compared to the control. Cortisol levels at three days post-weaning (DPW) were significantly lower in the HY group than the control group. Calves fed HY had significantly-higher serum levels of tumor necrosis factor-α and interleukin-1β at one DPW. The HY-fed calves also had higher concentrations of the acute-phase proteins, haptoglobin, serum amyloid A, and transferrin at one DPW. In addition, the diarrhea severity in HY-fed calves was milder after weaning compared to the control group. Our results indicate that HY supplementation reduces stress responses and may promote innate immunity in newly-weaned calves.